This invention relates to an improved method for amplitude versus offset analysis of common depth point seismic traces and more particularly to a method for correcting such traces for overburden attenuation affects.
Modern seismic prospecting techniques normally employ the common depth point, CDP, or common midpoint, CMP, techniques to improve signal-to-noise ratio. In these methods, seismic signals are generated sequentially at each of a number of points along a seismic prospecting path while reflections are recorded at all of the points following generation of each signal. The recorded signals are then organized into gathers of traces each corresponding to a common depth point or common midpoint. That is, all of the traces in a gather occur from source receiver pairs equally spaced about the point in question along the prospect path. The basic purpose of this exploration technique is to allow the signals within each gather to be combined to improve signal to noise ratio. Due to the different path lengths involved in each source receiver pair, corrections must be made to the individual traces within the gather to place them in alignment before stacking. These corrections are known as normal move out, or NMO, corrections.
Various attempts have been made to analyze CDP data in order to estimate various characteristics of subsurface formations which may be used to predict the hydrocarbon bearing potential thereof. One important method is known as the amplitude versus offset, AVO, technique in which the variation in amplitude of signals reflected from given subsurface interfaces is analyzed for changes relating to the angle of incidence or offset between source and receiver pairs. In order to properly perform such analysis, all other sources of amplitude variation should be removed first. Ideally, one would like to be able to measure the AVO characteristic of each rock interface individually. In practice, however, this is not possible since seismic waves must pass through overburden layers before they reach each particular interface in question. In so doing, they undergo angle dependent absorption and reflection, which attenuates the strength of seismic energy reaching the particular interface in question. The waves reflected from the interface also suffer the same types of losses in their journey back to the surface of the earth. Errors in estimating such absorption and reflection effects can generate erroneous amplitude changes which are greater than the actual variations resulting from the change in angle of incidence.